152 



UNDULATORY FORCES. LIGHT PHOTOGRAPHY. [PRINTING FKOM NEGATIVES. 



remainder. The two solutions may then be mixed 

 together for use, and be filtered if necessary : or, the 

 bath may be made by dissolving 300 grains of nitrate of 

 silver in six ounces of distilled water. Into this place a 

 oollodionised plate, and leave it in the bath for twelve 

 hours. Sufficient iodide will be removed from its sur- 

 face to iodise the bath. 



The plate being made sensitive, is then to be exposed 

 in the camera, as when positives are being produced ; 

 but, as a general rule, the exposure should be of longer 

 duration. Here, again, definite directions are impos- 

 sible, because of the great variety of contingencies, such 

 as weather, amount of light, temperature, <tc., which 

 have to be taken into account. 



After due exposure, the plate is taken into the dark 

 room to be developed. A different kind of solution is 

 employed for this purpose, to that used in developing 

 positives, although the process, so far as manipulation 

 is concerned, is similar. Much care, however, must be 

 taken to produce the best result. The process should 

 be conducted slowly, and the effect of the developing 

 solution must be constantly watched. When the ope- 

 rator conceives that a good depth of tone, and decision 

 of light and shade is obtained, the process must be 

 immediately stopped, and the plate should then be 

 abundantly washed with cold water, preparatory to fix- 

 ing. We annex recipes for various developing solutions 

 as follows : 



Developing Solution* for Negative Picture*. 



Protosulphate of iron . . .1 ounce. 



Acetic acid ..... 12 drops. 



Distilled water .... 1 pint. 

 Or 



Pyrogallic acid .... 6 grains. 



Glacial acetic acid . . . .40 drops. 



Distilled water . . . .10 ounces. 

 This solution requires filtration before use. 

 Another solution, as follows, may be used: 



Protosulphate of iron . . .6 grains. 



Glacial acetic acid . . ... 5 drops. 



Distilled water .... 1 ounce. 

 Or 



Pyrogallic acid .... 4 grains. 



Glacial acetic acid . . . .60 drops. 



Distilled water .... 4 ounces. 

 Our readers will here perceive the extraordinary 

 latitude which is given in the choice of developing 

 solutions. We must, however, repeat, that whilst we 

 have employed each of the above, and scores of othert>, we 

 do not venture to say which is the best, because the cir- 

 cumstances under which they are employed vary so much. 

 Presuming that the operator has succeeded in properly 

 developing the picture, the next process, after careful 

 washing, is that of fixing it. This is done by im- 

 mersing it in a bath, or pouring over it a solution of 

 hyposulphite of soda, which may be of the strength of 



Hyposulphite .... 3 ounces. 



Distilled water . . . . 10 

 The picture must then be well washed in cold water, as 

 already recommended, for positives, and so all traces of 

 soluble matter must be removed. 



As a negative plate has to undergo what we call 

 comparatively rough usage, its collodion side must be 

 protected in some way to prevent its destruction. This 

 is done by coating it with a thin layer of varnish. The 

 plate must first be gently heated, and the vaniish is 

 then to be poured on so as to cover the coated side on 

 every part, most evenly. After the whole surface is 

 covered with the liquid, the superfluous quantity should 

 be drained off, by inclining one end of the plate until it 

 is pretty well cleaned. The plate is then restored to its 

 horizontal position, and left to dry, being carefully 

 covered over to prevent dust or dirt falling on its 



The following varnishes have been recommended for 

 this purpose, as buitable on account of their trans- 

 parency and other qualities ; but the operator can pur- 

 chase those of a proper kind at the photographic che- 



mists. The object to be kept in view is, to have a 

 varnish which shall be transparent, and not liable to 

 crack when it becomes dry. 



Negative Varnishes. 



Mastic varnish, diluted till of a thin creamy con- 

 sistency. 



Sclmee's varnish, which consists of white lac dissolved 

 in spirits of wine. 



Copal vaniish, of the consistency of thin syrup. 

 It sometimes happens that the depth of shade, in some 

 negatives, does not sufficiently contrast with the lighter 

 portions of the picture: the result is, that a good print 

 can scarcely be obtained from such. To deepen those 

 tones, we have tried baths or solutions of the bichloride 

 of mercury, sulphate of iron, gallo-sulphate of iron, 

 iodide of potassium, <fec. , of various strengths. A writer 

 on the subject, in the Photographic Newt Almanac, re- 

 commends the following solution, which can be applied 

 after the picture has been fixed : namely 



Iodine 1 grain. 



Iodide of potassium ... 1 ,, 



Water 1 ounce. 



The picture is to be covered with this solution, and 

 to be left for five minutes : the process may be con- 

 ducted in daylight. A fresh development in the dark is 

 then required, by means of the following solution : 

 Pyrogallic acid .... 2 grains. 

 Glacial acetic acid. ... 15 



Distilled water 1 ounce. 



This is to be poured over the plate a few times. To 

 this solution may then be added a few drops of a twenty- 

 grain solution, to the ounce of water, of nitrate of siver. 

 This new solution is then to be poured over the plate. 

 The iodine and pyrogallic solutions may be alternately 

 used, until sufficient intensity has been obtained. The 

 picture is then to be well washed, and afterwards 

 varnished. The inventor of the plan states, that, as an 

 intensifying process, it is to be preferred to that of the 

 bichloride of mercury ; and our own experiments con- 

 firm that view. The student will arrive at some suc- 

 cessful results by taking the observations we have made, 

 rather as a guide to his own experiments, than as final 

 dicta. In hurried and unregistered trials with various 

 solutions, we have occasionally hit upon successful re- 

 sults. We, however, do not recommend such a course 

 to our younger readers ; because, although permissible in 

 those having experience, it engenders a careless habit in 

 the beginner, which, of all things, is most to be avoided 

 in scientific pursuits. 



We have thus given, so far as we can judge, the most 

 definite directions for obtaining good negatives. Our 

 next course will be to show how such may produce 

 positives by means of prepared paper of various kinds. 



PRINTING ON PAPER FROM NEGATIVES. 



WE have stated, that the chief use to which negative 

 pictures are applied, is that of producing au almost 

 indefinite number of positive copies ; and this is effected 

 by means of sensitive paper, which the solid parts of the 

 negative picture protect from the action of the chemical 

 rays of light, whilst the lighter portions permit that 

 influence to have a greater effect. The transparent parts 

 of the negative allow the freer transmission of light, and 

 thus we can obtain a picture presenting, so far as light 

 and shade are concerned, an exact copy of the original 

 We regret to say, that all attempts to copy the natural 

 colours of objects have hitherto failed. It has often 

 been announced that such has been accomplished ; but 

 hitherto no success has actually been attained, although 

 some approximation has been realised. A positive copy, 

 although precisely the reverse of a negative original, yet 

 has all its blemishes and excellences : hence it follows, 

 that unless a good negative is employed, a good positive 

 can never be printed from it. This fact the tyro in 

 photography will do well to bear in mind, as it will 

 induce care and caution in his attempts to produce the 

 negative in the first instance. 



